After you beat the game and let the game save after the ending credits, then the next time that you play, you will be in your room in Nuvema town. Go downstairs to find your Mom talking to... a clone of herself? It turns out that it's actually Looker, from Pokémon Platinum. After removing his disguise, he tells you that six of the Seven Sages of Team Plasma are scattered around Unova, and he asks you to help track them down and defeat them. Then he gives you the Super Rod, which he admits has nothing to do with the search for the Seven Sages.
Outside, Cheren and Bianca will meet up with you and talk to you. Bianca tells you of her plans to go to Black City (if you're playing Black) or White Forest (if you're playing White). Then Cedric Juniper will step outside and give each one of you the National Pokédex! Yay! Now you can catch the pokémon from other regions!
What you can do in the game now
Now that you have beaten the main game, there are still plenty of things left for you to do. Here is a list, which I have attempted to put in order from easiest to hardest.
- Catch pokémon from older games (check the list of pokémon locations)
- Fight trainers around Unova, all of whom have pokémon at level 60-65.
- Receive the Lustrous Orb, Adamant Orb, and Griseous Orb from the Shadow Triad on Marvelous Bridge.
- Visit White Forest or Black City, depending on your game version.
- Visit the Poké Transfer Lab to send pokémon from older games to your Black or White game. You need to have two DS systems to do this. Pokémon who are transferred via Poké Transfer can't be sent back.
- Catch Landorus by bringing both Tornadus and Thundurus to Abundant Shrine (you will have to trade one of them from another cartridge).
- Catch Volcarona in Relic Castle
- Catch Kyurem in Giant Chasm
- Receive HM06 Dive in Undella Town, and then dive for treasure, including the Arceus Plates.
- Find the Six Sages
- Battle Game Freak Morimoto in the Game Freak Building in the eastern part of Castelia City.
- Battle Cynthia in Undella Town
- Have rematches with the Elite Four, and battle the Champion afterward.
There is a whole eastern part of Unova that you haven't been able to access until now. You can reach it by either going east from Nimbasa City or east from Opelucid City. It doesn't really matter which way you go, since the trainers that you will encounter are all going to have pokémon of level 60-65. Wild pokémon in normal grass in these new areas will be a maximum of about level 50, while the ones that you find in dark grass and water environments will be level 55 and up.
If you're wondering where N went, find the Six Sages.